Applicant hereby claims foreign priority benefits under 35 U.S.C. xc2xa7 119 of corresponding Japanese patent application No. 2000-140877, filed May 12, 2000.
1. Field of the Invention
The present invention relates to a method for high concentration carburizing and quenching of steel which is suitable for a general machine part or a driving force transmitting part such as a gear, a bearing, a cam component for which high contact fatigue strength and wear resistance are required, and particularly suitable for such an automobile part subject to a possibility of being accompanied by hardness reduction caused by frictional heat due to rotation and sliding. The present invention also relates to a high concentration carburized and quenched steel part to which the method is applied.
2. Description of the Related Art
In general, a mechanical steel part in many cases must be provided with wear resistance with its surface hardness being increased and must keep its high toughness with its internal structure being made relatively soft. For this purpose, the steel is often treated with carburizing and quenching, or carbonitriding and quenching, for which carbon steel and alloy steel each having a carbon content of the order of 0.2% are used in such forms as case-hardened carbon steel, case-hardened alloy steel, machine structural steel, and machine structural alloy steel. Typical materials of such kinds of steel are chromium steel, chromium molybdenum steel, and nickel chromium molybdenum steel.
Incidentally, the above method for carburizing and quenching may be classified into a normal method for carburizing and a method for high-carbon carburizing (or method for high concentration carburizing). The normal method for carburizing is carried out in a carburizing atmosphere with a carbon concentration below that for Acm transformation. The method for high-carbon carburizing is for producing carbide with a carbon concentration in an atmosphere brought to a high level such that it is above the concentration corresponding to an intersection of the treatment temperature level with an Acm transformation curve in an iron-carbon system phase equilibrium diagram. When a high surface hardness is necessary, the latter method for high concentration carburizing is often employed. In the method for high concentration carburizing, the carbide is precipitated in network-like fashion along grain boundaries. A material with thus precipitated carbide has higher hardness and is excellent in wear resistance compared with a material treated with the normal method of carburizing, but it has a drawback of causing reduction in material strength.
In order to overcome the above drawback, it has been commonly practiced or carried out that the network-like carbide is granulated by making use of an A1 transformation. Namely, the temperature is reduced from the carburizing temperature (usually 900xc2x0 C. or above) down to the A1 transformation temperature or below, or is kept around the A1 transformation temperature for a long time to granulate the network-like carbide. The temperature is raised again thereafter for carrying out quenching treatment.
For example, U.S. Pat. No. 5,595,610 discloses the following method for producing case-hardened steel parts suitable for automobile parts in being particularly excellent in bending fatigue strength. Namely, according to the disclosed method, the steel parts are first carburized at 930xc2x0 C., for example, before being gradually cooled at a rate of, for example, 20xc2x0 C./minute for the subsequent heating held at 800xc2x0 C. Thereafter, the temperature is once reduced down to 750xc2x0 C. for being held at the temperature before quenching is carried out, by which the surface layer of the steel is made to have mixed structure mainly constituted of martensite and retained austenite.
Moreover, U.S. Pat. No. 4,913,749 discloses a method of producing rolling bearing elements in which the rolling bearing elements are case-hardened. According to the disclosed case-hardening process, the rolling bearing elements are carburized at 970xc2x0 C. before being cooled down to 300xc2x0 C. for being held at the temperature. The elements are thereafter heated up to 805xc2x0 C. for being quenched or up to 610xc2x0 C. for being annealed before being quenched at 805xc2x0 C.
Furthermore, Japanese Patent kohkai 117059/1999 discloses a composition of steel for preventing precipitation of cementite when carburized at elevated temperatures of 980xc2x0 C. or above together with the method for carburizing of the steel. The reference further discloses the following recent methods in the section of the prior art.
According to the disclosure, carburizing in recent years is carried out also by high temperature carburizing or vacuum carburizing. The high temperature carburizing is a method of carrying out RX gas carburizing at elevated temperatures of 950 to 1000xc2x0 C., which uses RX gas (endothermic gas) as a carrier gas together with an enriched gas such as butane gas. The vacuum carburizing is a method of carrying out carburizing and carbide diffusing in a reducing atmosphere for which hydrocarbon gas is decomposed under reduced pressure.
A special method for carburizing is also proposed, in which two or more carburizing cycles are repeated with one or more of the carburizing cycles carried out under an atmosphere with a carbon concentration being above Acm carbon concentration.
In addition to the above methods for carburizing, in order to make the network-like carbide less liable to be produced, a method of carrying out high concentration carburizing is proposed which uses a steel having special composition.
However, there were the following problems in the above methods for high concentration carburizing and high concentration carburized and quenched parts in the related art. 1) As described above, in the method for high concentration carburizing, it is necessary to prevent the material from having a reduction in strength by removing the network-like carbide. This, however, requires a high carburizing temperature and a complicated treatment process. In addition, in the method for high concentration carburizing, there is a problem in that the retained austenite is liable to be produced, which tends to reduce hardness of the material with accompanied reduction in wear resistance.
2) In the method for high concentration carburizing, it is also necessary to increase the carbon concentration in the atmosphere as described above. This, however, produces soot resulting in so-called sooting problem. The soot attached to furnace material of the treatment facility causes a problem of reduction in material life of the furnace, and the soot attached to the material to be treated causes a problem of loss of brightness of the parts.
3) In addition, for the high concentration carburized and quenched parts, it is required that they obtain high surface portion hardness, wear resistance, and fatigue resistance by a simplified treating method without using any steel having special composition as described above. In particular, for automobile parts used in applications in sliding contact with other parts at elevated temperatures, it is required that they are provided with not only, of course, high hardness, but also high or good resistance to softening, that is, that the part surface hardness at elevated temperatures is not lowered below the part surface hardness at room temperature.
According to one aspect of the present invention, a method for high concentration carburizing and quenching of steel by which the steel can obtain desired mechanical properties by a treatment process with lower carburizing temperatures and simpler operation than those in the related art without causing any problem of producing soot, is disclosed.
According to another aspect of the present invention, a high concentration carburized and quenched steel part which has excellent mechanical properties such as high surface portion hardness, wear resistance, fatigue resistance, and high or good resistance to softening, and which is particularly suitable for an automobile part, is disclosed.
The method for high concentration carburizing and quenching of steel according to the present invention is to be carried out by a treatment process including the steps of carbonitriding the steel at temperatures of from about 800 to about 880xc2x0 C., and subsequently quenching the steel at a temperature higher than the carbonitriding temperature. In the step of carbonitriding, the steel is treated with carbon (C) concentrations in a carburizing atmosphere of from about 0.7 to about 1.2 weight % carbon by weight of the steel and with from about 3 to about 8 volume % of ammonia gas (NH3) being added, for example. This dissolves network-like carbide in autstenite and, when the austenite is transformed into martensite by quenching, distributes approximately uniformly the carbide granules in the martensite.
The manner in which the foregoing and other aspects of this invention are accomplished will be more apparent by referring to the following description and appended drawings.